Research On Darkening Kinetics Characteristic Of Yb-doped Silica Fiber

High power Yb-doped fiber lasers(YDFL)are available with high efficiency,good beam quality and compact structure,which have been widely used in fields of industrial processing,deep-space optical communication,military,etc.However,YDFL suffer from photo-darkening(PD)effect,which are induced by pump photons,and radio-darkening(RD)effect,which are induced by ionizing radiation,when operated under harsh radiative environments.These two darkening effects can cause an obvious increase in fiber loss and a drastic decrease on fiber laser properties,but the darkening mechanisms have not been fully explained.This thesis mainly focuses on the darkening kinetics characteristic of YDF.Based on the darkening mechanism and self-build optical performance measurement setup.The purpose of this research work is to improve the radiation resistance performance of YDF,provided an essential guarantee for the space-based application of high power YDFL.Firstly,the research progress of darkening effect of YDF in the world was summarized,the existing problems were analyzed and effective methods for solving those problems were proposed.The influences of Yb ions,co-doping ions,and fiber structure on fiber properties were expounded.The absorption and luminescence characteristics of common defects were summarized,providing the basis for the identification and analysis of color centers(CCs)by fitting with Gaussians.Secondly,research methods on RD are analyzed,and a method is provided on measuring core absorption coefficient spectrum of YDF.The test approaches for RD characterized by the absorption spectrum and luminescence spectrum are suggested and the corresponding devices are designed.Based on the experimental data analysis,the bleaching of the radiation-induced absorption(RIA)by the ?-ray itself was concluded.The RD on influence the luminescence(including photo-luminescence and up-conversion luminescence)intensity of YDF was studied.It was proved that the reductions of total luminescence intensity with increasing total accumulated ionizing-radiation dose.Optical absorption spectra,photo-luminescence and up-conversion luminescence spectra had been employed to verify the hypothesis that there might be several reasons to cause RD.Thirdly,the possibility and way to improve the radiation resistance performance are discussed on the basis of CCs formation mechanism.The methods to mitigate RD effect were studied.A series of fibers are tested.The results indicate that Ce is a good choice.Ce ions is not only reduce the formation of CCs,but also beneficial for the recovery.Furthermore,the influence of pre-irradiation on the radiation resistance performance of YDF was investigated by measuring optical absorption.The results show that the RD resistance of YDF can be improved effectively via pre-treatment with high power laser(2-3 kW,976 nm).Finally,the intense photo-bleaching(PB)was found,when the irradiated YDF was pumped by 375 nm,which have potential to suppress the RD.The influence of Ce ions on PB was proved from the difference of irradiated Yb-doped and Yb/Ce co-doped fibers pumped by 375 nm.With these,two kinds of mechanisms were proposed to explain the PB of YDF.